Conservation of energy has become an objective of increasing concern in recent years and one obvious approach to that goal is to increase the efficiency of utilization of energy. Boilers and other heaters of liquid are so commonly used both in industry and residentially that any improvement in their efficiency would of course result in vast savings of energy.
It has long been recognized that greater efficiency in liquid heating can be had by increasing the area of heat-exchanging surfaces between the source of heat and the liquid to be heated. Various designs of liquid tanks have been proposed and one which has shown promise is an annular tank. Generally, in such designs, hot products of combustion in gaseous form are introduced into the cylinder formed by the interior wall of the annular tank. The hot gases pass over the interior wall which serves as a heat-exchange surface to heat the liquid in the tank. It has even been proposed that the gases then be redirected over the exterior wall of the annular tank, in which case, both walls serve as heat transfer surfaces. In some instances, fins, convoluted surfaces, or other area-increasing devices have been used to attempt to improve efficiency of heat transfer but such devices have been self-defeating to some extent because of their interference with the flow of hot gases, because of fabrication costs, and because of complexity of structure. Hence it is the primary object of the present invention to maximize the benefits and the gains in efficiency of heat transfer which can be realized by combining with annular tanks efficient sources of hot gases and heat transfer enhancers which are relatively simple and inexpensive and which operate to avoid the effects of boundary-layer interference with heat transfer.